Abstract

We have isolated a C. albicans gene, named FCR3 (for fluconazole resistance 3), based upon its ability to suppress the FCZ hypersusceptibility of a Saccharomyces cerevisiae mutant strain (JY312) lacking the transcription factors Pdr1p and Pdr3p. The FCR3 ORF (1200 bp) encodes a 399 amino acid protein containing a basic leucine zipper (bZip) domain. Fcr3p displays the highest level of sequence homology with the S. cerevisiae Yap3p protein (34% identity, 45% similarity). We had previously shown that deletion of the PDR5 gene encoding a multidrug transporter completely abolished the ability of FCR3 to suppress the FCZ hypersusceptibility of JY312, suggesting that FCR3 confers FCZ resistance by activating PDR5 expression. We show here that the beta-galactosidase activity of a PDR5 promoter-lacZ construct in JY312 is increased two-fold upon FCR3 overexpression, demonstrating that FCR3 regulates PDR5 at the transcriptional level. We also show that FCR3 overexpression not only suppresses the hypersusceptibility of JY312 to 4-nitroquinoline-N-oxide (4-NQO) but also confers higher levels of resistance to this compound as compared to the wild-type KY320 strain. Since PDR5 is not involved in 4-NQO resistance, this result indicates that FCR3 can also activate the transcription of other genes that can confer 4-NQO resistance. Finally, Northern blot analysis indicates that FCR3 encodes a single 2.4 kb RNA transcript in C. albicans, suggesting that the FCR3 mRNA contains long 5‘ and/or 3’ untranslated regions. The nucleotide sequence of the FCR3 gene has been deposited at GenBank under Accession No. AF342983.